1. Field of the Invention
This invention relates to valve apparatus used in subsea well test safety equipment, and more particularly to a ball valve with the capability of cutting relatively large diameters of coiled tubing extending through the valve when the valve is closed in an emergency situation.
2. Description of the Prior Art
In the production testing of offshore wells, it is desirable to be able to quickly disconnect the production test string from the well in the event of an emergency such as adverse weather conditions. In making the quick disconnect, provision must be made for shutting in the well. This is commonly accomplished with a well apparatus referred to as a subsea test tree. A subsea test tree is a type of safety valve. Another device is a jack-up safety valve used on jack-up rigs which is also used to sever coiled tubing.
A typical example of a prior art subsea test tree is seen in U.S. Pat. No. 4,494,609 to Schwendemann. Quite often when the production test string is in place in a well, it will be necessary to run other tools down through the production test string and down through the subsea test tree. These other tools are typically run on a wireline or on coiled tubing.
It is common practice with subsea test trees such as that of Schwendemann, when an emergency situation arises, to close the ball valve member of the test tree while the wireline or coiled tubing still extends through the test tree, thus severing the wireline or coiled tubing by the shearing action of the ball valve member against its seat.
As procedures utilizing coiled tubing have evolved, the industry has moved toward use of larger diameters of coiled tubing. This presents an increased difficulty in severing the coiled tubing in emergency situations. That is, the introduction of larger diameter coiled tubing has initiated a need to increase capabilities in subsea well test safety equipment to insure that rig safety is not compromised when emergency closure of the safety valve is required during operations such as nitrogen jetting, acid spotting and fishing.
Various approaches have been suggested to improve upon the capability of a subsea test tree for cutting these larger strings of coiled tubing.
One approach is that shown in U.S. Pat. No. 4,009,753 of McGill et al., wherein a slot is cut in the lower portion of the spherical valve member so that when the ball valve moves to its closed position with a string of coiled tubing still in place, the lower portion of the coiled tubing string will be received in the slot of the ball valve and thus will not be placed in double shear type bending as the ball valve closes.
Another approach to the problem is seen in U.S. Pat. No. 4,160,478 to Calhoun et al. The Calhoun et al. device does not use a conventional spherical ball valve member, but instead uses a combined cutter/valve operator member mounted eccentrically within the housing and associated with a spherical seat surface which is also developed on an eccentrically positioned center.
Still another approach is presented in U.S. Pat. No. 5,284,209 to Godfrey which is specifically designed to cut larger strings of coiled tubing. This is accomplished without placing any recess in the spherical ball valve member, and thus has the major advantage of not weakening the ball by removing material from the ball. An eccentric recess is formed within the inner bore of the valve housing at a position diametrically opposed to the point where the spherical ball valve member closes upon coiled tubing to shear the coiled tubing. This allows a lower portion of a coiled tubing string to be received in the eccentric recess and thus substantially reduces the plastic deformation of the lower portion of the coiled tubing string as the ball valve member is closed to shear the coiled tubing. This significantly reduces the closing force necessary to cut a given string of coiled tubing, and thus allows a given subsea test tree design to reliably cut larger strings of coiled tubing than it otherwise could in the absence of the eccentric recess defined in the inner bore of the housing.
In each of the previous ball valve arrangements, a substantially cylindrical pin engages a slot in the ball to actuate the ball between its open and closed positions. This results in the contact between the pin and the wall surface of the slot being basically limited to a single line, and at one end of the rotation of the ball, even this line of contact is reduced because of the relationship of the pin with the curved outer surface of the ball. The shearing load necessary to cut large diameters of coiled tubing can result in damage to the ball or the pin because of the high loading resulting from this very small contact area.
The valve of the present invention solves this problem by providing a slot cut into the ball with a matching control pin which fits in the slot and is pivotally connected to a control frame. During an opening or closing cycle, the pin slides within the slot but maintains a relatively large, flat area of contact between the pin and ball, greatly reducing the stresses on these components. This allows greater loading without damage to provide the increased shearing loads necessary for cutting larger diameters of tubing. The valve of the present invention can also be easily retrofitted into old equipment, thereby increasing the tube cutting capabilities of such equipment. The design can also improve the operation of valves in which the ball is opened against differential pressure from below and also in situations where ball valves are used in sand service.